On the canvas put the two images together with one of the images selected click the gradient tool in the left-hand tools box make sure the colours are set to black and white, now go to the top and click the gradient box you should see white into transparent, click that and draw your gradient across one image.

On the canvas put the two images together with one of the images selected click the gradient tool in the left-hand tools box make sure the colours are set to black and white, now go to the top and click the gradient box you should see white into transparent, click that and draw your gradient across one image.